I’m sure that historians have pondered the first question at length, but I haven’t read their lucubrations. According to Wikipedia, the first definitive use of the wheel on transportation was in Mesopotamia around 3500 B.C. We don’t know how many times it was invented independently, but probably more than once (see below):
So, my first question is this: Why was the wheel not devised in the New World? The Americas had plenty of civilizations, including many Native American groups, and the Aztecs, Incas, and Maya as well as many other groups, but none of them had the wheel, with one exception (see below). Why? Further, the Diquis culture had stone spheres beginning about 300 A.D., so they certainly knew that something round could roll. But this wasn’t adapted for carts or other rolling entities. Yet the Incas are said to have used wooden rollers to roll large stones for their walls and cities. Why no wheels, then?
According to The Straight Dope (I just looked this up), there was one exception:
The wheel evidently was familiar to the ancient Mexicans, the only known instance of its having been invented independently of the Sumerian version. Unfortunately, it apparently never occurred to anyone at the time that wheels had any practical application, and their use was confined to little clay gadgets that are thought to be either toys or cult objects.
That link also gives you an explanation that Cecil Adams considers definitive, but I don’t know. See for yourself. I am guessing that Jared Diamond pondered this question in Guns, Germs, and Steel but I read it so long ago I can’t recall. Go to the link, read “the” answer to my question above, and see if you agree with Cecil.
My second question is this: How did our ancestors keep their fingernails and toenails at reasonable length?
I thought of this question while clipping my nails the other day, and thought, “Scissors and nail clippers, and even steel knives were not invented in fairly late in human history. But yet our ancestors did without them for millions of years. How did they keep their nails short?
Now you might say, “They didn’t need to: their nails wore down from hunting, gathering, and walking barefoot.” But I am not sure this is the case. How would walking barefoot wear down your toenails? And we know that, at least in modern society, if you don’t trim your fingernails and toenails, they get ungodly long (see below). Did the ancients use flint? And what did they do before they had flint implements? Or did they bite their fingernails?
Now we could surely answer this question by observing what hunter-gatherers do, if anything, to keep their nails short. But I am not going to look it up; I’d rather have readers speculate or, if they know the answer, tell me.
Below: a video showing what happens if you don’t trim your nails: here’s a man who didn’t trim the nails on one hand for 66 years. (He explains why.) He has, on that hand, the longest known fingernails in history.
Of course he had to cut his nails on his right hand so he could do stuff (and I’m betting he’s a rightie). Nobody would marry him, and you can imagine the trouble he had just living from day to day. It’s all in the video
At the end they cut his nails: